Assessment of myelopathy in cervical ossification of the posterior longitudinal ligament by MRI-assisted 3D measurement.

BACKGROUND: Ossification of the posterior longitudinal ligament (OPLL) is a 3-dimensional (3D) disease that causes cervical myelopathy. The conventional 2-dimensional (2D) measurement of OPLL has limitations in estimating cord compression and myelopathy. In this study, we attempted to use 3D computed tomography (CT) and magnetic resonance imaging (MRI) to measure the 3D occupying ratio of OPLL and investigate its significance in the assessment of spinal cord myelopathy. METHODS: Three-dimensional CT and MRI were performed in 50 patients with cervical OPLL at a neutral position before surgery. MRI was done to determine the extent of spinal cord compression. The CT data were saved in DICOM format and analyzed using Mimics 17.0. Then a 3D model of OPLL was semiautomatically segmented at a specific threshold. The following data were measured: diameter of the spinal canal, thickness of the OPLL, and 3D volume of the OPLL and spinal canal. The Japanese Orthopedic Association (JOA) score was used to assess the cervical spinal cord function. RESULTS: Pearson correlation analysis showed that both the occupying ratio and the 3D occupying ratio were significantly and negatively correlated with the JOA score. Multiple linear regression analysis indicated that only the 3D occupying ratio showed a significantly negative correlation with the JOA score, whereas age, sex, and the occupying ratio were insignificantly associated with the JOA score. CONCLUSIONS: The 3D occupying ratio of OPLL is a reliable indicator for assessing the severity of spinal cord myelopathy. MRI provides more details about cord compression, making the measurement more accurate and objective.